The host species for glochidia of unionid mussels were examined in Lake Anenuma, Aomori Prefecture. As a result, Anemina arcaeformis was recorded for the first time from Aomori Prefecture and was found to infect two host fish species (Gymnogobius castaneus and Tribolodon hakonensis). Sinanodonta sp. was found to infect four species (Hypomesus nipponensis, G. castaneus, Tridentiger brevispinis and T. hakonensis); Hyriopsis schlegeli infects three species (H. nipponensis, G. castaneus and T. brevispinis), and Inversiunio jokohamensis also infects three species (G. castaneus T. brevispinis and T. hakonensis). The fish species composition was investigated and the prevalence of infection by unionid mussel glochidia for each host species in Lake Anenuma was investigated. Hypomesus nipponensis was the dominant fish species. Sinanodonta sp. mainly utilized H. nipponensis, A. arcaeformis utilized two species (G. castaneus and T. hakonensis), and the two other unionid species (H. schlegeli and I. jokohamensis) mainly utilized G. castaneus.
Mytilus trossulus Gould, 1850 is a mytilid bivalve with a boreal distribution in the northern Pacific Ocean, northern Atlantic Ocean, and Baltic Sea. The distribution of M. trossulus in Japanese waters was hitherto believed to be restricted to the northernmost island of Hokkaido. However, we discovered dry specimens and dead shells of this species on the northern and central Japan Sea coasts of Honshu Island. Specimens were collected before 1936, before 1948 and in 1951 from Shikaura, Fukui Prefecture (35°56´N, 135°59´E), and were archived at the Fukui City Museum of Natural History as "Mytilus edulis Linnaeus 1758". Dead shells with rotten soft bodies were collected from the Kisakata sandy shore, Akita Prefecture (39°12´24˝N, 139°53´40˝E) on March 29, 2014. In addition, we found old records of the nonindigenous congener M. galloprovincialis Lamarck, 1819 in molluscan lists that were published in Akita, Niigata, Ishikawa and Fukui Prefectures from the 1930s to 1950s. This species was introduced to Japan before 1932 and appears to have been infrequently confused with M. trossulus. In 2007, 2010 and 2014, we conducted field surveys in the regions where the dry specimens and dead shells had been collected but found no M. trossulus specimens. In view of the results of the field surveys and water temperature regime in its distribution range, we believe that the dry specimens and dead shells had drifted from the more northerly Japan Sea coasts of Russia or Hokkaido. The old records of M. galloprovincialis in the molluscan lists may indicate the actual occurrence of the nonindigenous species during the early years of its invasion in Japan.